Reliable Change Index
Compute a practice-adjusted Jacobson–Truax reliable-change index from caller norms and criterion; not a diagnosis or a universal improvement cutoff.
Description
Reliable-change scores compare an observed score difference with estimated retest uncertainty.
Reliable-change scores compare an observed score difference with estimated retest uncertainty. Subtracting expected practice change adjusts the numerator.
The tool returns standardErrorDifference (standard error difference); reliableChangeIndex (reliable change index); exceedsCriticalValue (exceeds critical value). Input measurements and model parameters are supplied by the caller; the tool computes the stated summary rather than acquiring those measurements.
When to use Reliable Change Index
Use Reliable Change Index when you already have baselineScore, followupScore, normativeSd, testRetestReliability, expectedPracticeChange, absoluteCriticalValue and want standardErrorDifference, reliableChangeIndex, exceedsCriticalValue. This is useful for checking a supplied calculation, documenting a measurement convention, or comparing observations prepared on the same basis.
How to use Reliable Change Index and understand its results
- Enter baselineScore, followupScore, normativeSd, testRetestReliability, expectedPracticeChange, absoluteCriticalValue using the field table below. Match the units, ordering and data boundaries described in the calculation.
- Run the tool after checking the stated bounds and matching paired observations or categories. A malformed request gives a tool error rather than a usable measurement.
- Read standardErrorDifference, reliableChangeIndex, exceedsCriticalValue. Use the calculation and interpretation below to distinguish amounts, ratios, signs and decision flags where applicable.
- Review the supplied parameters and assumptions before using the result in a report or comparing it with another observation.
| Input field | Meaning | Value type and bounds | Requirement |
|---|---|---|---|
| baselineScore | Baseline score | number | Required |
| followupScore | Followup score | number | Required |
| normativeSd | Normative sd | number | Required |
| testRetestReliability | Test retest reliability | number | Required |
| expectedPracticeChange | Expected practice change | number | Required |
| absoluteCriticalValue | Absolute critical value | number | Required |
Require positive SD and critical magnitude, with reliability in [0,1). Flag absolute RCI strictly greater than the criterion.
SEdiff = normativeSD × sqrt(2(1−reliability)); RCI = (followup−baseline−practice)/SEdiff The symbols refer to the supplied quantities or generated values described above; unit conversions and comparison boundaries follow the stated implementation.
{
"input": {
"baselineScore": 10,
"followupScore": 14,
"normativeSd": 5,
"testRetestReliability": 0.92,
"expectedPracticeChange": 1,
"absoluteCriticalValue": 1.96
},
"output": {
"standardErrorDifference": 1.9999999999999996,
"reliableChangeIndex": 1.5000000000000004,
"exceedsCriticalValue": false
}
}Limitations and assumptions
Reliability and practice effects must fit the population and test. Equality is not flagged; the result does not diagnose cognitive decline.
Numerical inputs must be finite. The tool checks its declared input shape and calculation constraints; those checks do not verify the quality, calibration or representativeness of the supplied observations.
Alternative or Complementary methods
Cognitive Decline Slope: Fit longitudinal cognitive score change after caller-supplied visit-specific practice offsets; score direction and diagnostic interpretation are not assumed. Crawford-Howell t: Compare one case with a small normative control sample using Crawford–Howell t and n−1 degrees of freedom; no p-value, tail choice or clinical interpretation is invented.
Methodological background: Measures of scale: sample variance and standard deviation.1
References
-
Measures of scale: sample variance and standard deviation — NIST/SEMATECH
Similar or alternative tools
- Cognitive Decline Slope
Fit longitudinal cognitive score change after caller-supplied visit-specific practice offsets; score direction and diagnostic interpretation are not assumed.
- Crawford-Howell t
Compare one case with a small normative control sample using Crawford–Howell t and n−1 degrees of freedom; no p-value, tail choice or clinical interpretation is invented.